1. Field Of The Invention
With the advent of pre-cooked foodstuffs, foodstuff packaging laminates have been in remarkably increased demand, and various ones are being used. In particular, there is an increased demand for foodstuff packaging laminates which enable foodstuffs packed therein to be sterilized at high temperatures, such sterilization being referred to as "retort" and such packed and sterilized foodstuffs being called "retort foodstuffs" for convenience sake.
It is generally required that foodstuff packaging laminates have the following basic properties: (1) they must not be toxic and must be hygienically satisfactory since they contact foodstuffs directly; (2) they must be satisfactorily impermeable to gases or the like to permit foodstuffs packed therein to keep their flavor and taste unchanged for a long time (such impermeability being hereinafter sometimes referred to as a "barrier property"); (3) they must have satisfactory light-intercepting properties to prevent foodstuffs packed therein from degradation and denaturalization due to ultraviolet light radiation or the like; (4) they must have high mechanical strength and satisfactory impact strength, (5) they must be highly resistant to water and chemicals such as acids and alkalies and (6) they must have good heat sealability so that they can be thermally bonded under pressure in a very short time. In addition, (7) they are required to have good heat resistance and (8) not to lose bond strength when foodstuffs packed therein are subjected to sterilization in a retort. For example, the laminates must not suffer a diminution in any of the above properties when contacted with water, acids, alkalies, oils or the like at high temperatures, when they are subjected to retort conditions, i.e., high-temperature sterilization at usually 100.degree.-140.degree. C. for as long as one hour.
2. Description Of the Prior Art
A single packaging film alone will not meet such stringent requirements. Thus, composite films, i.e., laminates, are used as foodstuff packaging materials. The components of the composite materials for packaging foodstuffs include polyolefin, polyamide, polyester and aluminum and, in many cases, combinations of an aluminum substrate such as foil or sheet having particularly excellent barrier properties with a hygienically-acceptable polyolefin film are used as retort-proof packing or packaging materials for foodstuffs. Aluminum foils are generally used for lamination with polyester films or the like. Among polyolefin films, high density-polyethylene, polypropylene, ethylene-propylene copolymers and polybutene are preferred as retort-proof packing materials for foodstuffs because of their heat resistance.
It is generally known that high density-polyethylene, polypropylene, ethylene-propylene copolymers, polybutene and other polyolefins are useful as foodstuffs packaging materials because of their excellent sanitary properties. However, these materials exhibit poor adherence to other materials because of their high crystallinity and non-polarity. To eliminate thee drawbacks, the above polyolefins are subjected to various chemical and physical treatments, or to ultraviolet irradiation, electronic beams or the like. More particularly, chemical treatment of the polyolefins with sulphuric acid-chromate is certainly effective if conducted at elevated temperatures; however, this treatment is not efficiently conducted since it is of the wet type and corrodes the reaction apparatus. Therefore, this treatment is now used only for basic studies in attempts to improve the adhesiveness of polyolefins. Polyolefins copolymerized with .alpha.,.beta.-ethylenically unsaturated carboxylic acids, such as acrylic acid, methacrylic acid and maleic anhydride, as well as esters thereof, are used to attempt to improve adhesion for bonding polyolefin to other materials. In this case, however, either only a very low bond strength is obtained or heat and pressure are required for an excessive length of time to attain satisfactory bond strength. In addition, primers are used in many cases to accelerate adhesion. Corona discharge-treated polyolefin films are widely used for industrial purposes but, in this case, the improvement in bond strength is limited and an adhesive is additionally used. There are few industrial processes for modifying or reforming polyolefin itself to make it highly adhesive. In fact, polyurethane resins and the like are widely used as adhesives in cases where a highly secure bond is necessary between polyolefin films and other materials.
However, polyurethane resins are said to have the following disadvantages: (1) it is probable that the unreacted diisocyanate and polyol, as well as monomers and low molecular weight polymers remain in the polyurethane resins. These substances migrate to the packaged foodstuffs thereby raising food contamination problems; (2) It usually takes about one week at ambient temperaure or at least one day at 50.degree.-60.degree. C. to obtain, by aging, a practically useful bond between metallic and/or plastic (resinous) food packaging films; (3) Foaming is encountered, thereby producing nonuniform bond strength throughout the bonded portion and seriously affecting the acceptability of the resulting product. Although the polyurethane resins have the aforesaid disadvantages, they are still in use since no other materials useful as an adhesive for polyolefin and aluminum are now available.
Thus, there is a need for producing polyolefin-aluminum food packaging laminates having excellent, virtually instantaneous bond strengths, and resistance to deterioration or delamination under retort conditions without imparting deleterious substances to the packaged food.
It is also known that polyolefin resins incorporating a metal compound are used in lamination in the field of building or construction materials and packing materials. In addition, the metal ion-crosslinked polyolefin resins are considered as a kind of thermoplastic resin (ionomer) wherein the long-chain molecules are connected to each other by means of ion linkage. Structurally, the ion linkages between the long-chain molecules are produced with aid of the monovalent or polyvalent metal cations and the carboxyl groups of long-chain molecules.
The compositions and uses of conventional polyolefin resins are disclosed in literature as indicated below.
In U.S. Pat. No. 3,264,274, R. W. Rees discloses ionic copolymers obtained by reacting a copolymer of an .alpha.-olefin and an ethylenically unsaturated monocarboxylic acid with a monovalent to trivalent metal ion and also discloses ion-crosslinked copolymers obtained by reacting a copolymer of an .alpha.-olefin and an ethylenically unsaturated dicarboxylic acid with a monovalent metal ion. In addition, he teaches that the ion-crosslinked copolymers are useful as adhesives and they may be laminated on paper, metal foils and plastics. However, he also states that it is inappropriate to react the copolymer of .alpha.-olefin and .alpha.,.beta.-ethylenically unsaturated dicarboxylic acid with the polyvalent metal ion.
In Japanese Patent Gazette, Patent No. 19238/77, K. Shirayama et al. teach that a resin composition is useful as an adhesive in forming laminates of polyolefin with a metal, the resin composition being prepared by reacting a crystalline polyolefin with an usaturated aliphatic carboxylic acid and/or anhydride thereof as well as with the oxide and/or sulphate of a metal of Groups IIa, IIIa and IVb of the Periodic Table at a temperature higher than the melting point of the polyolefin.
In the Japanese Patent "Laying-Open" Gazette, Ser. No. 37494/73, K. Shirayama et al. disclose a polyolefin composition prepared by reacting polyolefin with an unsaturated aliphatic carboxylic acid and/or anhydride thereof as well as with the hydroxide or alcoholate of a metal of Groups Ia, IIa, IIIa and IVb of the Periodic Table at a temperature higher than the melting point of the polyolefin, and they also disclose that the polyolefin composition so prepared exhibits particularly excellent adhesion to polyolefin moldings, aluminum, iron, copper, zinc and the like and teach that the composition may be used as an adhesive in laminating a synthetic resin with a metal.
In Japanese Patent Gazette, Patent No. 17971/72, I. Aijima et al. disclose a process for a feasible thermoplastic monoolefin polymer by reacting a thermoplastic monoolefin polymer with (a) a radically polymerizable carboxylic acid, (b) a radical initiator and (c) the oxide, hydroxide or carbonate or lithium, potassium, sodium, magnesium, calcium, zinc, aluminum or silicon, at an elevated temperature in the presence or absence of a solvent or medium. The feasible thermoplastic monoolefin polymer so obtained may be used in the production of injection molded articles, vacuum molded articles and the like without a decrease in impact strength and creep characteristics.
In Japanese Patent "Laying-Open" Gazette, Ser. No. 27580/74, I. Iwami et al. disclose laminates in which a metal-containing copolymer is interposed between the metal and the ethylenic-copolymer, the ethylenic copolymer being a copolymer of the ethylene, an .alpha.,.beta.-ethylenically unsaturated carboxylic acid, an .alpha.,.beta.-ethylenically unsaturated carboxylic acid metal salt and, if desired, an .alpha.,.beta.-ethylenically unsaturated carboxylic acid ester. The laminates so disclosed are used in communication cables.
In Japanese Patent "Laying-Open" Gazette, Ser. No. 74583/73, T. Fujimoto et al. teach packing materials which are sterilizable at a high temperature under pressure and have as the inner layer a film prepared from a composition consisting of high-density polyethylene and ionomer. The Gazette also teaches that an ionomer sold under the trademark Surlyn (produced by E. I. Du Pont De Nemours & Co.) may be used as a retort-proof packing material for foodstuffs. However, the ionomer-incorporated film is laminated by means of dry lamination using a urethane type adhesive.
In any event, it is not known that polyolefin resins prepared by combining together a heat-resistant polyolefin, maleic anhydride and aluminum hydroxide, are very useful in the production of retort-proof packing materials for foodstuffs, having excellent, virtually instantaneous bond strength and being capable of production at a high rate of speed.
It is necessary that foodstuff packaging laminates be manufactured at a high production speed (that is, they must be bonded together virtually instantaneously and they must have high bond or adhesive strength before and after being heated at retort conditons). They must not decrease in bond strength during prolonged storage in contact with various foods after having been retorted. Further, they must not produce any appreciable extracts or materials which migrate into the food.
Therefore, the primary object of this invention is to provide a process for preparing retort-proof, polyolefin-aluminum foodstuff packaging laminates which can be produced at a high production rate with virtually instantaneous bond strengths and with excellent bond strengths even under retort conditions, and which laminates permit the storage of foodstuffs therein without significant loss of bond strength.